Use attributes for native module ABI and link name

This patch changes how to specify ABI and link name of a native module.

Before:
  native "cdecl" mod llvm = "rustllvm" {...}

After:
  #[abi = "cdecl"]
  #[link_name = "rustllvm"]
  native mod llvm {...}

The old optional syntax for ABI and link name is no longer supported.

Fixes issue #547

src/comp/lib/llvm.rs

@@ -110,7 +110,9 @@ const LLVMRealULE: uint = 13u;
 const LLVMRealUNE: uint = 14u;
 
 #[link_args = "-Lrustllvm"]
-native "cdecl" mod llvm = "rustllvm" {
+#[link_name = "rustllvm"]
+#[abi = "cdecl"]
+native mod llvm {
 
     type ModuleRef;
     type ContextRef;